When taking blood samples for test purposes, whole blood is generally drawn into an evacuated fluid or blood collection container or tube, and the tube is subsequently centrifuged to separate the blood into its major phases, that is, its relatively light or low density phase, serum or plasma, and its heavy or high density cellular phase. Blood phase separators or partitioning devices have been used to provide a partition or barrier between the separated phases until the light phase is removed for clinical testing. Some proposed partitioning arrangements include the use of a sealant or a gel-like material having a specific gravity intermediate the specific gravities of the light and heavy blood phases so that during centrifugation and phase separation, the sealant automatically flows to the interface of the two phases and forms a semi-solid partition between them. Various gel-like materials or sealants are now well known. For example, in U.S. Pat. No. 3,852,194, a thixotropic, gel-like mixture of silicone and hydrophobic silicon dioxide powders is used to form a partition between separated phases. In U.S. Pat. Nos. 4,021,340; 4,088,582 and 4,055,501, mixtures including a liquid polybutene polymer and silicon dioxide powders are used as gel-like phase partitioning materials.
There are certain problems associated with the use of gel-like sealants as phase barriers. In some cases, there is the possibility that the sealant will form a barrier too early so that red cells are trapped in the light phase, serum or plasma, thereby causing contamination of the light phase and inaccurate clinical test results. In some constructions, the sealant travels through the heavy cellular phase colliding with cells, and this is believed to cause cell hemolysis which adds to the contamination of the light phase. On the other hand, in some cases, there is the possibility that the sealant will not flow soon enough after centrifugation has been initiated or that it will remain in its initial location and fail to form a barrier or an adequate one. In general, because of variations in temperature and sealant viscosity, speed of the centrifuge, sealant aging, and other factors, the use of gel-like sealant materials as phase partitions have not been entirely reliable.
In copending applications, U.S. Ser. Nos. 31,816 now abandoned, 31,817, now abandoned and 31,818, filed Apr. 20, 1979, now U.S. Pat. No. 4,246,123, and which have the same assignee as this application, standpipes are disclosed for conveying sealant through the high density phase for reducing red cell hemolysis and also to meter the flow of gel through the pipe so that it does not reach the interface too early. While these devices are improvements, the flow characteristics of the sealant through the standpipes may still vary due to one or more of the above-mentioned possible variations.